1. Field of the Invention
The present invention relates to a deflection yoke for use in color cathode ray tubes to produce a magnetic field so as to deflect the electron beams.
2. Description of the Prior Art
In general, a deflection yoke used in color cathode ray tubes with an in-line type electron gun is designed to produce a pin-cushion shape horizontal deflection magnetic field and a barrel-shape vertical deflection magnetic field. This type of yoke, commonly called a self-convergence type, is advantageous in that it does not require a complex convergence circuit. On the other hand, this type of yoke is likely to involve misconvergence of the pattern as shown in FIGS. 14(a), (b) and (c), owing to an erroneous assembly of the electron gun and/or the yoke itself. The misconvergences of the pattern shown in FIGS. 14(a) and (b) are caused owing to an axial misalignment, but can be fixed by correcting the tilt of the deflection yoke or alternatively either by adjusting a differential coil connected to the horizontal deflection coil or a differential resistance connected to the vertical deflection coil. In contrast, the misconvergence shown in FIG. 14(c) is caused owing to a rotational misalignment, to which the above-mentioned correction methods cannot be applied. In FIGS. 14(a), (b) and (c) the line R represents a lateral red line, the line B represents a lateral blue line, b, g and r represent the sources of electron beams for blue, green and red, respectively.
Japanese Laid-Open Patent Publication No. 60-91536 discloses a means of correcting a misconvergence, which includes a two-pole magnetic ring, a four-pole magnetic ring, and a six-pole magnetic ring, and additionally, another quadripolar magnet ring so as to correct a misconvergence due to a rotational misalignment. There is another method for correcting misconvergence, which includes a magnetic ring provided in the other deflection coil as a magnetic shield whereby an equivalently reversed misconvergence is deliberately caused.
The known correcting methods of misconvergence described above have disadvantages, in that, with the first method the two quadripolar magnet rings must be alternately rotated until the correction is effected. The repetition of the alternate rotation of the magnet rings is likely to unfavorably affect the convergence characteristics in the center of the fluorescent screen. In the second method it is difficult to correct the whole fluorescent screen.